1. Field of the Invention
The present invention relates to a pressure valve for equalising pressure differences between a closed container or tank and the surrounding atmosphere. In particular, the invention relates to a pressure valve for use in connection with a cargo hold or a tank in a maritime carrier. However, the invention also lends itself for use in connection with stationary tanks or tanks supported by wheels.
2. The Prior Art
For instance, U.S. Pat. No. 5,873,384 to the present applicant, which is incorporated herein in its entirety by the present reference, teaches a pressure valve of the kind featured in the preamble to claim 1.
When a tank is loaded with a given volume of liquid per time unit, pressure valves of this type open when the gas pressure within the tank reaches a pre-defined opening pressure and a gas volume per time unit which corresponds to the product volume loaded per time unit plus any evaporation from the liquid surface in the tank is then to be blown out. The blown-out volume per time unit depends on the flow resistance of the valve and of the pressure prevailing within the tank. The maximum blown-out volume per time unit minus any evaporation, which may occur without the pressure within the tank exceeding a given safety threshold value Pmax, is designated the capacity of the pressure valve. The capacity of the pressure valve determines the maximally allowable loaded volume per time unit. When, upon interruption of the loading, the pressure within the tank drops to a certain value, i.e. the closing pressure of the pressure valve, which is lower than the opening pressure, the pressure valve closes. In the known pressure valves, the closing pressure is considerably lower than the opening pressure. Additionally any increase or drop in the ambient temperature during storage or transport of eg petroleum products or chemicals leads to boiling-off which may bring about pressure changes within the container, which in this case will cause the pressure valve to open and close in fundamentally the same manner as during loading and unloading.
Another use of pressure valves applies when, for safety reasons, it is required to supply an inert gas, such as nitrogen, to the free space of the container above the product being transported. For instance, the product may be an inflammable liquid or a material, such as oranges and lemons, that gives off inflammable gasses, following protracted storage. The inert gas is automatically kept at a predetermined pressure that may be above the atmospheric pressure, but below the opening pressure of the pressure valve. In this case any rise or a drop in the temperature of the surroundings will give rise to a corresponding change in the pressure of the inert gas within the container, whereby the pressure valve must open.
In the known pressure valves, it has been found that the difference between the opening pressure of the valve and the closing pressure, due to the design of the pressure valves, is too large for the pressure valves to be optimally suited for use in the transport of certain materials. It is thus the object of the present invention to provide a pressure valve of the kind described above, wherein the difference between the opening pressure and the closing pressure can be reduced to about 10%–20% of the closing pressure. When eg fruit is transported, it is hereby possible to obtain a reduction in the amount of inert gas that must be supplied when the ambient temperature drops, following initial blow-out through the pressure valve.
This is obtained by the features given in the characterising part of claim 1.
By the embodiments featured in the dependent claims, a particularly convenient configuration of the characteristic curve of the pressure valve during the course of the opening is obtained. In particular by the measures featured in claim 10, a considerable reduction of the difference between the opening pressure and the closing pressure is obtained. The relations given in claim 10 should be complied with throughout the entire area of movement of the lifting plate.
The invention will now be explained in further detail with reference to the embodiment shown in the drawings.